When a computer graphics image created by a computer, electronic data created by page description language or image data created by an external input device such as a digital camera is printed out on a recording medium by an image forming apparatus such as a color printer, the usual practice is to form a color image using a plurality of colorants (toners or inks, etc.) of such colors as cyan (C), magenta (M), yellow (Y) and black (K).
In order to faithfully reproduce the color of an original color image in an image forming apparatus for thus forming the color image, the image signal is corrected by executing a masking operation, which takes the spectral characteristics of the colorant into consideration, and color correction processing based upon a profile created for every output device for the purpose of accommodating for a difference between color spaces in different types of machines. There are also cases where an image is formed without applying any modification at all to signal values of each of the colors in the electronic data.
In general, when an image is formed on a recording medium using colorants such as toners or inks in an image forming apparatus, there is a possibility that output results will not be acceptable or that the image forming apparatus itself may sustain physical damage depending upon the type of recording medium or process conditions involved in image formation.
In a case where processing based upon amount of colorant (toner) that has exceeded a suitable amount is executed in an electrophotographic printer, poor transfer and fixing occur if the amount of applied toner for each color component becomes excessive, and this can lead to damage to the printer engine and to a decline in image quality ascribable to “toner spray”. For example, it is known that when the amount of applied toner of each of the color components for cyan (C), magenta (M), yellow (Y) and black (K) becomes excessive in an electrophotographic color printer, “toner spray” occurs particularly in characters and thin lines if the image to be printed is a character image or computer graphics image. Accordingly, if there is a large amount of applied toner, image processing for converting the value of data representing each color to a low value is executed.
Processing for limiting the number of colors of output image data (the overall signal value of each of the colors C, M, Y, K) upon thus taking into consideration the total amount of colorant that is output in dependence upon the characteristic of the output device is referred to generally as toner reduction processing (see the specification of Japanese Patent Application Laid-Open No. 2002-166602), particularly in an electrophotographic image forming apparatus.
On the other hand, in a case where the amount of applied toner is large when forming part of a photographic image, the intended density can better be obtained by printing the image using the image data as is without execute toner reduction processing, this being achieved in a way that will not damage the printer engine.
Accordingly, in the conventional image forming apparatus, whether or not tone reduction processing is executed is changed over from print job to print job. The option to make the changeover for every print job is decided by a parameter that turns a toner-reduction processing mode ON or OFF (i.e., that turns toner reduction ON or OFF).
With the conventional image forming apparatus, however, toner spray in characters and thin lines will become conspicuous if a document that is a mixture of characters or thin lines and photographic images is printed with the toner-reduction processing mode left OFF (i.e., with toner reduction left OFF).
On other hand, if printing is carried out with the toner-reduction processing mode left ON (i.e., with toner reduction left ON), there are instances where the density of photographic images will be inadequate owing to toner reduction processing.